The present invention relates to electric glass-melting techniques and more particularly those in which the conductivity of the molten glass is used in order to develop the power needed to melt the raw materials.
For a long time, glass-producing plants operating on large quantities of material have been provided with melting furnaces powered with fossil fuel, especially gas. This was in particular the case for high capacity continuous production plants supplying, for example, flat glass or bottle glass. In these large furnaces, electricity, when it was used, was mainly used as a local top-up in order to maintain the temperature of the glass in the cooler regions, or, outside the furnace, on its path toward the location of transformation or else in order to develop certain convection movements assumed to enhance the homogenization, the refining or the transport of the molten material.
Electric melting itself firstly appeared in small units for which considerable flexibility of conditions of use seemed necessary. The fluctuations in energy costs and the progressive mastering of some problems of a technological nature led more recently to the development of large production units in which the whole melting process, with the exception of the start-up, takes place while having recourse to electricity. This development requires extremely tricky technological problems to be solved.
Hence, in particular to avoid the question of oxidation of the electrodes at the surface of the molten bath, it was proposed to immerse them completely. This is the solution retained, for example, in French patent application published under No. FR-A-2 552 073. In this document, the electrodes are arranged vertically in the bath from the floor of the furnace. In other embodiments, electrodes passing through the lateral walls of the furnace are also found.
Independently of the advantages that it affords with respect to problems of corrosion, the immersion of the electrodes also make possible a convenient and very uniform power supply to the surface of the bath with a raw material composition. The constitution of a relatively thick layer of composition to be melted, floating on the molten bath, is in fact useful for several reasons. On contact with the molten bath, it forms the permanent reserve of material needed for continuous operation. It also protects the molten bath from high energy loss by convection in contact with the atmosphere and especially by radiation.
Although the furnaces of the type described in the aforementioned document find very wide industrial applications, they do not necessarily make it possible to respond as well as possible to all the requirements encountered in practice. By way of example, it is desirable, in some cases, and with the obvious aim of limiting investment costs, to transform plants operating with burners by retaining as much as possible of the existing elements and especially the refractory materials constituting the tank. A transformation of this sort is not always possible when it involves implanting electrodes in the floor or in the lateral walls of the furnace.
Furnaces whose electrodes are immersed by means of the floor or the walls of the vessel offer limited possibilities for controlling the electrodes. Although they lead to very satisfactory performances for certain conditions, they adapt less well to frequent and/or substantial modifications of these operating conditions.
It is for this reason that furnaces called xe2x80x9cfurnaces with plunging electrodesxe2x80x9d have sometimes been preferred. Thus, French patent FR 2 599 734 describes a furnace of this type in which the electrodes are arranged and distributed so as to optimize the efficiency of the furnace and above all its flexibility of use.
Conventionally, this type of furnace comprises a floor made of a refractory material, four lateral walls or upstands, an upper roof, a glass composition supply means and an output channel for the molten glass.
It may also comprise side panels which can be moved vertically, intended to isolate the bath from the surrounding atmosphere at certain moments of the glass formation process.
The thermal insulation of the bath of glass is in fact a major problem in glass-making furnaces.
The present invention provides a solution to this problem which is both new and original.
In fact, it allows a selective and controlled insulation of the furnace, which furthermore is multi-purpose and efficient.
These characteristics cannot be found anywhere in the prior art which, at best, relates to a system of the xe2x80x9call or nothingxe2x80x9d type with regard to insulation of the furnace. Moreover, experience has shown that when the lateral panels which can be moved vertically are closed, the insulation of the furnace is not correctly provided, in particular because of the clearances between the panels.
Advantageously, the leaktightness of the furnace is perfectly preserved according to the invention.
Thus, the subject of the invention is an electric glass-melting furnace comprising a floor made of a refractory material, four lateral walls or upstands, an upper roof, a glass composition feed means and an output channel for the melted glass.
According to the invention, it further comprises a set of elements pivoting about horizontal axes and placed at the periphery of the furnace between at least one of the lateral walls and the roof.
The modularity of the furnace insulation is thus produced as a result of there being a set of pivoting elements and not a single element per side.
Furthermore, the melting elements are (flat) juxtaposed panels, the thicknesses of which are cut vertically so as to create a perfect seal between two contiguous panels.
An arrangement of this sort guarantees a seal between each of the panels and therefore an overall seal comparable to that obtained with a single panel.
According to a particular feature of the invention, the furnace further comprises a set of cables, pulleys and winches intended to make said pivoting elements pivot individually or in a group.
Thus, depending on the type of use chosen, or even the time of the glass manufacturing process, a larger or smaller number of pivoting elements will be able to be open, which makes it possible to adjust the thermal insulation of the furnace.
According to another particular feature of the invention, the furnace further comprises a mechanism for locking the pivoting elements in the open position.
Furthermore, the furnace according to the invention may comprise a system for unlocking the pivoting elements.
This aspect of the invention provides perfect security of operation of the pivoting elements and hence control of the overall insulation of the furnace.
According to an interesting aspect of the invention, the pivoting elements are mechanically reinforced by elements such as rods implanted within their thickness. The rods are preferably metallic.
According to one embodiment of the invention, the pivoting elements are arranged over at least three lateral sides of the furnace.
The present invention may be used in a furnace of the plunging electrode type. In this case, provision may be made to arrange at least one pivoting element opposite each plunging electrode.
In compliance with one characteristic of the invention, the furnace further comprises measuring sensors which can be implanted inside the furnace, on at least one lateral wall, and at least one pivoting element is placed opposite at least one measuring sensor.
Furthermore, the furnace comprises a system intended to lift simultaneously all the pivoting elements placed on at least one of its sides.
This possibility, preferably reserved for the side called a furnace-charging side, that is to say through which the distributor of the composition is introduced, ensures speed when carrying out panel movements, which is highly appreciated by the users.